This invention relates to electronics, and more particularly, to compositions useful for producing conductor patterns adherent to substrates.
Conductor compositions which are applied to and fired on dielectric substrates (glass, glass-ceramic, and ceramic) usually comprise finely divided inorganic powders (e.g., metal particles and binder particles) and are commonly applied to substrates using so-called "thick film" techniques, as a dispersion of these inorganic powders in an inert liquid medium or vehicle. Upon firing or sintering of the printed film, the metallic component of the composition provides the functional (conductive) utility, while the inorganic binder (e.g., glass, Bi.sub.2 O.sub.3, etc.) bonds the metal particles to one another and to the substrate. Thick film techniques are contrasted with thin film techniques which involve deposition of particles by evaporation or sputtering. Thick film techniques are generally discussed in "Handbook of Materials and Processes for Electronics", C. A. Harper, Editor, McGraw-Hill, N.Y., 1970. Chapter 12.
Thermistors are typically ceramic resistor bodies whose electrical resistance is temperature dependent. Those whose resistances decrease with an increase in temperature are referred to as negative temperature coefficient (NTC) thermistors, while those whose resistances increase with an increase in temperature are referred to as positive temperature coefficient (PTC) thermistors. Thermistor bodies are generally bodies of fired ceramic semiconductors. In the case of the NTC thermistors, the latter are usually one or more metal oxides of a large group of metal oxides known to have semiconductive properties, some of the more commonly used being the oxides of metals such as manganese, nickel, cobalt, iron, zinc, vanadium, zirconium,cerium, chromium and uranium. The PTC thermistor bodies generally are fired alkaline earth titanates which have been rendered semiconducting by the substitution of, for example, a small amount of a lanthanide (atomic number 57-71) or yttrium to yield compounds having the general formula A.sub.1-x B.sub.x TiO.sub.3 where A is Ba, Ca, and/or Sr and B is the substituted atom. Often the titanate is lanthanium-doped barium titanate, Ba.sub.1-x La.sub.x TiO.sub.3. Thermistors of both NTC and PTC types must be provided with electrically conductive contacts to which circuit leads may be attached.
The conductive contacts or electrodes applied to thermistor bodies should be low resistance, essentially ohmic contacts, especially for PTC bodies. Silver compositions are widely known and used for providing fired-on conductive contacts or electrodes on ceramic objects. However, most commercial silver compositions do not provide low resistance, ohmic contacts when fired onto semiconductive PTC bodies the reason apparently being that sufficient oxygen from the PTC body penetrates through the coating during firing to provide an oxidized nonconducting or barrier layer between the fired-on coating or electrode and the semiconductive substrate. Short U.S. Pat. No. 3,547,835 (issued Dec. 15, 1970 and incorporated by reference herein) provided silver conductive compositions which minimized the penetration of oxygen from the semiconducting body into the silver coating during firing, by adding certain amounts of aluminum to the silver composition. This material has been widely used commercially, but (as disclosed at col. 3, line 73 to col. 4, line 1 of U.S. Pat. No. 3,547,835) its fired coatings are not directly solderable. Of course, leads must be soldered onto the electrode to form a functional device. Hence a silver coating free of aluminum is applied over the Ag/Al coating of Short to permit soldering.
Low resistance contacts for semiconducting ceramics are reviewed by J. W. Fleming et al., Ceramic Bulletin 55, 715-6 (1976) and H. M. Landis, Journal of Applied Physics 36, 2000-2001 (1965). A two-step process for making contacts on semiconducting ceramics (flame-spray deposition of a layer of Al, then a layer of Cu) is disclosed in Kourtesis et al. U.S. Pat. No. 3,676,211.
There is a need for a silver material which can be applied to a semiconducting body in a single step and fired to produce a low-ohmic electrode which in both adherent and solderable, eliminating the significant expense of application of a second silver layer over the initial fired silver coating.